Magnetoresistors are devices whose resistance varies with a magnetic field applied to the device, and are therefore useful as magnetic field sensors. The magnetoresistor is useful for position sensing applications and may be useful for a variety of other applications, such as brushless motors or magnetic memory storage devices. Initially, magnetoresistors were believed to be best formed from high carrier mobility semiconductor materials in order to obtain the highest magnetic sensitivity. Therefore, the focus was on making magnetoresistors from bulk materials that were thinned down or films having sufficient thickness to exhibit a high average mobility. The use of bulk materials that were thinned down has the disadvantage that the process of thinning the body of the material results in defects in the surface of the body which adversely affects the operating characteristics of the body, such as lowering the electron mobility. Also, this process is relatively time-consuming and expensive to carry out. Forming the magnetoresistor of a thick film or substrate creates problems with the resistance of the film. Very large currents are needed to obtain a significant voltage drop across the device, and large amounts of power are then dissipated in the device.
In order to remove the problem of thick film magnetoresistors, attempts have been made to form the magnetoresistor from a thin film. However, the problems which arise from thin films of a magnetoresistor material on a substrate are illustrated in FIGS. 1 and 2. FIG. 1 shows the electron areal density vs. thickness of a film, and FIG. 2 shows the electron mobility vs. thickness of the same film. The film used was of indium antimonide (InSb) epitaxially deposited on a substrate of gallium arsenide (GaAs) to a thickness of one micrometer. To obtain the data for these graphs, the film was etched in a solution of by volume 1 part HNO.sub.3 and 8 parts lactic acid in several steps. After each etching step the measurements were made with Hall effect measurements. The zero thickness of the film is the interface of the indium antimonide film with the substrate. FIG. 1 shows that the areal carrier density for the material does not have an intercept at zero for zero thickness. This implies that the carrier density is not homogeneous in the film, and that the carrier density is higher near the interface of the film and the substrate and/or at the surface of the film. Many of these excess electrons probably are at the interface of the film and the substrate since crystallographic defects are known to induce electrons in indium antimonide. Secondly, FIG. 2 shows that the electron mobility is very poor near the interface of the film and the substrate. To the extent that the excess electron density is in the poor mobility region near the substrate, the average electron mobility in the film is reduced. Since the sensitivity of magnetoresistors to magnetic field depends on the second power of carrier mobility, the sensitivity of these devices is reduced by lower mobilities.
We believe that lower carrier mobilities near the interface of the film and the substrate are caused in large measure by the large crystalline lattice constant mismatch between high electron mobility materials, such as indium antimonide, and suitable substrate materials, such as gallium arsenide, which is about 14%. For practical purposes, this is difficult to eliminate since there are not many materials commercially available which can be used for the substrate of this device. Therefore, it is desirable to be able to modify the film used for the active layer of a magnetoresistor or other similar magnetic field sensor to improve the carrier mobility of the device.